This invention relates to a passive shim arrangement for increasing the uniformity of a magnetic field. More particularly but not exclusively, it relates to a passive shim arrangement for use in a nuclear magnetic resonance apparatus.
A nuclear magnetic resonance (NMR) apparatus requires a magnetic field with a high degree of uniformity. The main magnetic field in an NMR apparatus is generated by a cylindrical coil. As the main magnetic field is usually not adequately uniform for the purposes of NMR, it is necessary to employ some correcting device to increase the uniformity. A commonly-used device for correcting a magnetic field is a magnetic member, referred to as a passive shim, which is disposed inside the cylindrical coil. Typically, a plurality of passive shims are secured with a bonding agent to the inner surface of a support cylinder disposed inside the coil.
Presently, there are no quantitative rules on how to best position passive shims inside an NMR apparatus, and the technician who is installing the shims must rely on his own experience and intuition. As a result, the shim configuration often does not maximize the uniformity of the magnetic field, and the characteristics of the field may vary greatly from apparatus to apparatus. In addition, as the passive shims are installed in the NMR apparatus by bonding, they cannot be easily disconnected from the support cylinder, and it is difficult to adjust the positions of the shims.